Control circuit

ABSTRACT

A temperature sensitive responsive loop and a heater element control loop are functionally connected by a gate control loop combining to form a three loop temperature control circuit. The temperature sensitive responsive loop is magnetically coupled to the gate control which is coupled across a gate controlled switching element in the heater control loop.

United States Patent 11 1 Rieman Mar. 5, 1974 CONTROL CIRCUIT [75]Inventor: Willis E. Rieman, Skaneateles, N.Y.

[73] Assignee: Gulf & Western Systems Company, New York, N.Y.

221 Filed: Aug. 22, 1972 [21] Appl. No.: 282,828

Related u.s'. Application Data [63] Continuation-impart of Ser. No.212,016, Dec. 27,

[52] US. Cl. 219/490, 219/511 [51] Int. Cl. 1105b 1/02 [58] Field ofSearch... 219/490, 494, 497, 499, 501,

[56] References Cited UNITED STATES PATENTS 3,548,157 12/1970 Lauck219/501 3,588,447 6/1971 Mills 219/501 Primary ExaminerBemard A.Gilheany Assistant ExaminerF. E. Bell Attorney, Agent, or FirmMeyer,Tilberry & Body [57] ABSTRACT A temperature sensitive responsive loopand a heater element control loop are functionally connected by a gatecontrol loop combining to form a three loop temperature control circuit.The temperature sensitive responsive loop is magnetically coupled to thegate control which is coupled across a gate controlled switching elementin the heater control loop.

10 Claims, 4 Drawing Figures CON TROL LOOP CONTROLLED PAINTED 3.795.190

SHEET 1 [IF 2 TEMPERA TURE COIL 20 7 CONTROL CORE NH/c SENSOR ADJUSTABLESENSA um 26 I SWITCH f 5 I9 I 25 sauo BALANCE STA TE CONTROL w/nw m; I Ac A c HEATER g? souRcE 1 ELEMENT OFF 29d E; E l

25 26 f" I W HEA TER ELEMENT M88 HEA COIL SENSOTR CORE 5 20 l CONTROLCIRCUIT BACKGROUND OF THE INVENTION This application is acontinuation-in-part of US. Patent application Ser. No. 212,016 filedDec. 27, 1971.

The present invention relates to temperature sensitive heater elementcontrol circuits. More particularly the present invention is a triplelevel, temperature sensitive, adjustable, heater element control circuitemploying magnetic coupling between the first and second levels, andgating control coupling between the second and third levels forcontrolling, on a positive on or off" basis, the heater element of anoven or furnace.

The present temperature sensitive heater element control circuit hasadvantages over other circuits performing somewhat similar functions inthat the present circuit is much less complex than normally used, isvery stable and requires no adjustment after initial adjustment of thecircuit components has been made. In addition to these advantages thecomponents of the circuit which are not temperature responsive, aresubstantially temperature stable so thatthe circuit is insensitive toambient temperature changes and in addition, is very inexpensive tobuild. The present control circuit has been found to work very well withtrouble-free, long life expectancy, without gradual degeneration of thecomponents of the circuit.

In its disclosed form the present invention includes a three loop orthree level circuit arrangement in which the first loop or level is atemperature sensitive and responsive loop. This loop includes a heat ortemperature sensor or responsive means, for example a thermistor, whichis located in the temperature controlled environment. This may be anoven (as represented inthe drawings) or some other environment such as aroom or structure of any place or thing or mass in which it' is desiredto control the environmental temperature, or the temperature of themass. Another component in the first loop is a temperature control,which in the present circuit is an adjustable resistor or potentiometer,which may be calibrated in degrees of heat. Another component of thefirst loop is a magnetic force generator, such as a coil. The first loopis driven by a direct cur rent (DC) which is applied to the loop at alltimes during the time the circuit is turned on. The resistanceinductancevalues of this circuit are such that when the circuit is in a balancedcondition the active resistance value of the circuit is such thatthecurrent flowing through the coil drives the coil to generate amagnetic force of predetermined intensity. The magnetic force is thecoupling between the first and second loops of the circuit.

The third loop is the heater element control loop which is a controlledloop, from a functional sense. The controlled loop includes a gatedswitching element, the heater element or heater element control coupledin series with an alternating current ,(AC) sufficient to drive theheater element or the heater element control. The gated switch may, forexample be a pair of back-toback silicone controlled rectifiers (SCRS)or a triac, with positive on/off control over the electric power drivingthe heater element.

The second loop may be referred to as a gate control loop. This circuitincludes a pair of contacts, for example a reed relay, and a resistanceconnected in series with the reed relay across the gated switch,therefore serving as a gate control for the gated switch. The pair ofcontacts are magnetic sensitive, the second loop being magneticallycoupled to the first loop functionally the pair of magnetic sensitivecontacts are posi tioned in the magnetic field generated by the coil.The second loo'p.is coupled to the third loop via its connection acrossthe gated switch of the third loop.

The magnetic sensitive contacts of the second loop are physicallycontrolled by the intensity or magnitude of the magnetic force generatedby the coil while the gated switch is controlled by the physicalposition (opened or closed) of the pair of magnetic sensitive contactsto the extent that when the contacts are closed the gating circuit ofthe gated switch is closed thereby turning the switch on thuselectrically completing the driving circuit for the heater element inthe third loop. The intensity or magnitude of the magnetic forcegenerated by the coil is a function of the DC input, the amount of turnsin the coil and the active resistance in the first loop of the circuit.The magnetic force may be changed by holding the DC input stable andmaintaining the number of turns in the coil the same and chang ing theactive resistance in the first loop. The resistance in the first loop ofthe circuit may thus be changed by a change in the environmenttemperature thereby causing a change in the active resistance of theheat sensor or thermistor or temperature responsive resistor or, by

manual adjustment of the adjustable resistance or potentiometer.

Therefore, it is an object of the present invention to provide animproved temperature sensitive heater element control circuit.

Another object is to provide an improved temperature sensitive heatcontrol circuit having components, other than the heat sensingcomponent, which are insensitive to ambient temperature change.

A further object is to provide an improved temperature sensitive, triplelevel heating element control circuit having trouble-free, long lifeexpectancy.

These and other objects will become apparent when reading the followingdetailed specification in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of the present invention;

FIG. 2 is a functional diagram helpful in understanding the invention;

FIG. 3 is a simplified circuit diagram of the preferred form of thepresent invention; and, I

FIG. 4 is a simplified circuit diagram showing another embodiment of thepresent invention.

Referring particularly to FIG. 1, it will be seen that an alternatingcurrent (AC) source 10 is provided for circuit operation. Some of theoutput of the AC source is rectified, in a conventional AC to DCrectifier, into a direct current (DC) 1 1. Other of the output of the ACsource is used to drive the heater circuit. The presented embodimentemploys-electric energy to drive an electric heating element. In thisarrangement the heater element 12 is energized by the AC source 10 underthe control of a slide state switch (55s) 26. This AC source could bethe usual volt AC source or a 220 volt AC. The electric energy could besingle phase or multiphase energy. The type of electric energy usedwould depend upon the characteristics of the circuit components in theAC loop. In addition,'the type of rectifier 11 would also depend uponthe type of AC used.

The heat sensor, block of FIGS. 1 and 2, is represented as a thermistor15 in the circuit diagram of FIG. 3. The thermistor 15 is connected toone terminal of the DC 11, for'example the positive output or terminal.FIG. 2 represents that the thermistor 15 is positioned in the controlledenvironment, here represented as an enclosed and insulated chamber 16,such as an oven, for example formed by a door 17 and the enclosingsides, top, bottom and end, 18. In the present arrangement, a thermistorhaving'a positive coefficient is employed, i.e. a thermistor whichincreases in resistance as the temperature of the environment in whichit is positioned, increases.

It should be noted that the same components are given identicalidentification throughout the drawings.

The temperature control-adjustable, block 19, may be a potentiometer(FIG. 3) and may be remotely positioned or may be adjacently positioned,as for example,

on the exterior of the environmental chamber (FIG. 2). The potentiometer19 may be calibrated in degress of heat, such as in Fahrenheit orCentrigrade or intensity of heat, such as warm, medium, hot, very hot.Block 20 may be referred to as a duplex block since this blockrepresents a coil 21 around an air core 22 in which is positioned amagnetic sensitive switch (mss). The other end of the coil is connectedto the return or negative supply or terminal of the DC. This is one loopor circuit open and at such threshold value of magnetic intensity, andabove such threshold value, the reed relay contacts reverse theircondition, for example close. A resistance 25 in series with reed relayadds balance and stability to the reed relay contact circuit, reducingcontact migration between the two contact points 'and reducing arcingduring movement of the contacts. The reed relay contacts areelectrically isolated from the CONTROL LOOP, these contacts and resistor25 forming part of what may be referred as a GATE LOOP since thesecomponents are connected across a gated switching component such as apair of back-to-back silicone controlled rectifiers (SCRS), 26 or atriac so as to provide level and may be referred to as the CONTROL LOOPof the triple level control circuit.

Functionally, the rectified DC is maintained at a substantially stablelevel using conventional AC to DC rectification technology. As thethermistor 15 changes resistance due to a change in the environmentaltemperature in the oven, the current through the coil 21 changes therebychanging the intensity ofthe magnetic field generated about the coil.Any change in the electrical parameters of the components of the CONTROLLOOP will cause a change in the intensity of the magnetic fieldgenerated by the coil, thus effecting the magnetomotive force presentaround the coil. Assuming the DC values remain constant and the numberof turns on the coil 21 remains constant, then, the value of activeresistance in the temperature control 19 may be changed, by adjustmentwhether manual or automated for example, or the value of activeresistance of the thermistor 15 maybe changed by a change in theenvironmental temperature. A change in the active resistance in thefirst loop of the circuit will cause a change in' the amount of currentflow through the coil 21. A change in the current flow through coil 21will change the amount of intensity of the magnetic force or fluxgenerated by the coil 21.

Within the core 22 or positioned within the magnetic field generated bythe coil 21 is a set of magnetic sensitive contacts such as a magneticresponsive reed relay having at least two mating contacts which respondat a predetermined magnetic threshold, i.e. change condition (fromnormally opened to closed, from normally closed to opened, and fromopened to normally closed or from closed to normally opened), when asufficient magnetomotive force is present and/or is changed tointensity.

The magnetic responsive reed relay (not shown) may have thresholdcharacteristics with respect to the intensity of the magnetic -forcegenerated by the coil 21. In this respect the sensitivity of the reedrelay may be such that below a predetermined condition, for example SCRSor the triac function as a solid state switch which is in the CONROLLEDLOOP of the triple level control circuit. When the contacts of the relay22 are closed, the solid state switch 26, is closed (or the gate isopened) to let current through to drive the heater element 12. A fuse 28is represented in the circuit, of the CONTROLLED LOOP and an on/off"switch 29 may be used to turn the entire circuit on or off," as desired.

If-the heat energy were other than electricity, as for example oil heator gas heat, the heating element could be controlled by a contact 14,controlled by a relay 13 which relay would be coupled in the controlledloop instead of the heater element. The heater element 12 would beremoved from the circuit at terminals 32 and 33 and the relay 13 wouldbe connected to the terminals 32 and 33. The relay l3 and the contact 14could be remote from the oil or gas heater unit and could be used in theoil heater circuit or gas heater circuit thereby providing positiveon/off control of the. oil heater unit or the gas heater unit. Analternative ar rangement would be to connect the on/off electric controlcircuit of the oil or gas heater to the terminal 32 and 33 directly andthereby control the current supply as shown.

In the circuit arrangement of FIG. 3, it will be recog- I with variableresistance means 15. Electromagnetic coil 21 may be defined as a firstswitch operating means. The gate loop of FIG. 3 may be defined as a second loop. The magnetic sensitive switch in this second loop defines afirst switch means which is positioned for operation by the first switchoperating means defined by coil 21. This defined first switch means isselectively movable between open and closed conditions by the firstswitch operating means. The controlled loop of FIG. 3 having heaterelement 12 therein may be defined as a third loop. Solid state switchingdevice 26 connected in series with heater element 12 in this third loopmay be defined as a second switch means. The second switch means definedby solid state switching device 26 includes a gate which is connected inseries with the first switch means in the second loop. This gate meansmay be considered a second switch operating means in the second loop fortriggering the second switch means defined by solid state switchingdevice 26.

In accordance with another arrangement, coil 21 and solid stateswitching device 26 of FIG. 3 are replaced with bimetallic elements asshown in FIG. 4. In this arrangement, first, second and third electricalloops A, B and C are provided. First loop A includes variable resistancemeans having automatically variable resistance characteristics withvarying temperature for passing electrical current therethrough at avalue proportional to its temperature. Variable resistance means 15 maybe a thermistor having a negative temperature coefficient. However, inthe preferred arrangement, variable resistance means 15 is of the typehaving a positive temperature coefficient. That is, an increase intemperature produces an increase in resistance so that current flowdecreases with increasing temperature. Such resistors are formed ofmetallic alloys, such as nickel alloys, and are very stable.

First loop A includes an electrical resistance heating element 42connected in series with variable resistance means 15. Electricalresistance heating element 42 may be defined as a first switch operatingmeans.

. Second loop B includes a first switch means which is operated betweenopen and closed conditions by first switch operating means 42. Thisfirst switch means includes a pair of normally opened contacts 44 and 46of an inexpensive switch. Contact 44 is connected for movement with abimetallic element 48. Bimetallic element 48 forms a pan of the firstswitch means and is positioned in heat transfer relationship with firstswitch operating means 42. Second loop B further includes second switchoperating means 50 defined by an electrical resistance heater connectedin series with the first switch means defined by contacts 44 and 46, andbimetallic element 48.

Third loop C includes a snap-acting switch 52 connected in series withheating means 12. Snap-acting switch 52 is normally open and ismechanically linked 'as at 54 with bimetallic element 56. Snap-actingswitch 52 and bimetallic element 56 may be defined as second switchmeans which is selectively movable between open and closed conditions inresponse to second switch operating means 50. Bimetallic element 56 ispositioned in heat transfer relationship with second operating means 50.

. Third loop C is supplied from a high voltage power source, while firstand second loops A and B are supplied with low voltage direct current.With such an arrangement, the switch containing contacts 44 and 46 maybe very inexpensive because arcing is no problem with the low voltagepower source. Switch 52 is preferably of the snap-acting typeto preventarcing and buming of the contacts due to the high voltage power supply.

In accordance with one arrangement, a control means in the form of arotatable knob 60 is provided for adjusting the operating temperaturewhich is being controlled. In the arrangement shown, knob 60 may begraduated in degrees Centigrade or Fahrenheit and connected withbimetallic element 48 for adjusting its position to vary the deflectionwhich must take place before contacts 44 and 46 are closed. In thearrangement shown and described, knob 60 is rotated to the desiredtemperature setting and the main power source is turned on. Current thenflows through first loop A to heat the first switch operating meansdefined by electri cal resistance heating element 42. The mass whosetemperature is being sensed by variable resistance means 15 is very lowso that a high current flow will occur to rapidly heat element 42.Bimetallic element 48 will then deflect in response to heat from heatingelement 42 and close contacts 44 and 46. Current will then flow throughsecond loop B to heat the second switch operating 'rneans defined byelectrical resistance heating element 50. Bimetallic element 56 willthen deflect in response to heat from heating element 50 and closeswitch 52 through linkage 54. When switch 52 closes, full power will besupplied to main heating element 12. As the temperature of the massbeing heated by main heating element 12 increases, the resistance ofvariable resistance means 15 increases so that the amount of currentpassing therethrough decreases. As the temperature of the heated massapproaches its desirable value, the current flowing through variableresistance means 15 becomes so low that heating element 42 transfersinsufficient heat to bimetallic element 48 for maintaining it in itsdeflected position closing contacts 44 and 46. Thus, when the massreaches its desired temperature, switch contacts 44 and 46 open todeenergize heating element 50. Bimetallic element 56 then cools to openmain snap-acting switch 52 and deenergizes main heating element 12. Asthe temperature of the mass begins falling below its desired value, theresistance of variable resistance means 15 decreases so that sufficientcurrent again flows therethrough for heating element 42 so that it cantransfer sufficient heat to bimetallic element 48 to deflect for closingcontacts 44 and 46. Heating element 50 then heats bimetallic element 56so that it deflects to close snap-acting switch 52. The circuit willthen cyclically operate for maintaining the temperature of the mass atits desired value. The first switch operating means defined by heatingelement 42 is responsive to variations in electrical current flowingthrough variable resistance means 15 for cyclically operating the firstswitch means defined by bimetallic element 48 and contacts 44 and 46.The second switch operating means defined by heating element 50 isresponsive to cyclical operation of contacts 44 and 46 for cyclicallyoperating the second switch means defined by switch 52 and bimetallicelement 56 for controlling operation of main heating means 12. It willbe recognized that the same cyclical operation occurs in the circuit ofFIG. 3. The first switch operating means defined by coil 21 isresponsive to variations in electrical current flowing through variableresistance means 15 for cyclically operating the first switch meansdefined by the magnetic sensitive switch positioned in the second orgate loop. The gate for solid state switching device 26, which definesa'second switch operating means, is responsive to cyclical operation ofthe first switch means defined by the magnetic sensitive switchfor'cyclically operatingthe second switch means defined by solid stateswitching device 26 for controlling operation of main heating means 12.

For some applications, it may be possible to eliminate second loop B,along with heating element 50, bimetallic element 56 and contacts 44 and46. Bimetallic element 48 would then be linked directly with switch 52.

At very low temperature settings, bimetallic element 48 would haveinsufficient deflection tocyclically operate snap-acting switch 52.Operation of conventional snapacting switches require more travel thanwill take place at low temperature setting for bimetallic element 48 ina small control arrangement of the type contemplated. In the preferredarrangement described, which includes second loop B, contact 44 may besoldered or brazed directly onto one side of bimetallic element 48 fordirect cooperation with contact 46. No snap-action is required due tothe low voltage power supply and the switch will operate at alltemperature settings.

It will be recognized that the various switch operating means and switchmeans defined in the circuits of FIGS. 3 and 4 may be interchanged. Forexample, heating element 42 in the first loop of FIG. 4 may be replacedby coil 21 of FIG. 3. The first switch means defined by contacts 44 and46 along with bimetallic element 48 would then be replaced by themagnetic sensitive switch of FIG. 3. The remainder of the circuit inFIG. 4 could be unchanged. Likewise, it will be recognized that it ispossible to replace solid state switching device 26 of FIG. 3 withheating element 50, bimetallic element 56 and snap-acting switch 52.Various other combinations will also occur to others skilled in the art.

The main heating means has been shown as an electrical resistance heater12. However, it will be recognized that heating element 12 may bereplaced with an electrical servo valve through which combustible gas isfed to a gas burner for heating the mass. Such a valve, in combinationwith the burner to which it supplies fuel, would then define a part ofthe heating mean which is controlled by the improved control circuitdescribed and claimed. Therefore, when a heating means is defined, itwill be recognized that this includes either an electrical heatingelement or an electrical servo valve or the like which forms part of agas-fueled heating means. I

First loop A includes terminals 64 and 66 which may be connected withany suitable source of AC or DC voltage in a known manner. Second loop Bincludes terminals 68 and 70 which may also be connected with anysuitable source of AC or DC voltage. Third loop C has terminals 72 and74 which are preferably connected with an AC voltage source, althoughthey may be connected with a DC voltage source when the third loop isused for controlling a valve through which combustible fuel is fed to aburner.

There has been described herein a triple level, temperature sensitiveadjustable heating control circuit, with suggested modification forcontrolling various forms of heater elements or burners. Although theinvention has been described with reference to certain preferredembodiments, it is obvious that equivalent alterations and modificationswill occur to others skilled in the art upon the reading andunderstanding of this specification. The present application covers allsuch equivalent alterations and modifications and is limited only by thescope of the claim.

Having thus described my invention, I claim:

1. A temperature control circuit for controlling operation of heatingmeans for heating a predetermined mass to a desired temperaturecomprising; a first elec trical loop including automatically variableresistance means having automatically variable resistancecharacteristics with'varying temperature for passing electrical currenttherethrough at a value proportional to its temperature, said variableresistance means being positioned in heat exchange relationship withsaid mass, first switch operating means in series with said variableresistance means in said first loop, a second electrical loop includingfirst switch means having open and closed conditions and positioned foroperation by said first switch operating means in said first loop,second switch operating means in series with said first switch means insaid second loop, a third electrical loop including heating means forheating said mass, second switch means in series with said heating meansand having open and closed conditions and positioned for operation bysaid second switch operating means, said first switch operating meansbeing responsive to variations in electrical current flowing throughsaid variable resistance means for a cyclically operating said firstswitch means, and said second switch operating means being responsive tocyclical operation of said first switch means for cyclically operatingsaid second switch means to control operation of said heating means.

2. The circuit of claim ll wherein said first switch operating meanscomprises a first resistance heater and said first switch means includesa first bimetallic element positioned in heat transfer relationship withsaid first resistance heater.

3. The circuit of claim 2 wherein said second switch operating meanscomprises a second resistance heater and said second switch meansincludes a second bimetallic element positioned heat transferrelationship with said second resistance heater.

4. The circuit of claim 3 and further including adjustable control meansfor adjusting the temperature at which said first switch means iscyclically operated by said first switch operating means.

5. The circuit of claim 1 wherein said second switch operating meanscomprises a second resistance heater and said second switch meansincludes a second bimetallic element positioned in heat transferrelationship with said second resistance heater. Y 6. The circuit ofclaim l and further including adjust able control means for adjustingthe temperature at which said first switch means is cyclically operatedby said first switch operating means.

7. A temperature control device for controlling operation of heatingmeans for heating a predetermined mass to a desired temperaturecomprising; automatically variable resistance means having automaticallyvariable resistance characteristics with varying temperature for passingelectrical current therethrough at a value proportional to itstemperature, said variable resistance means being positioned in heatexchange relationship with said mass, first switch operating means inseries with said variable resistance means, first switch means havingopen and closed conditions and being positioned for selective operationbetween said open and closed conditions by said first switch operatingmeans, second switch operating means in series with said first switchmeans for operation in response to opening and closing of said firstswitch means, second switch means having open and closed conditions andbeing positioned for selective operation between said open and closedconditions by said second switch operating means, said second switchmeans being connected in series with said heating means.

8. The device of claim 7 wherein said first switch operating meanscomprises an electro-magnetic coil and said first switch means comprisesa magneticallyoperated switch.

9. The device of claim 7 wherein said first switch operating meanscomprises a first resistance heater and said first switch means includesa first bimetallic element positioned in heat transfer relationship withsaid and said second switch means includes a second bimefirst resistanceheater. tallic element positioned in heat transfer relationship 10. Thedevice of claim 7 wherein said second switch with said second resistanceheater. operating means comprises a second resistance heater

1. A temperature control circuit for controlling operation of heatingmeans for heating a predetermined mass to a desired temperaturecomprising; a first electrical loop including automatically variableresistance means having automatically variable resistancecharacteristics with varying temperature for passing electrical currenttherethrough at a value proportional to its temperature, said variableresistance means being positioned in heat exchange relationship withsaid mass, first switch operating means in series with said variableresistance means in said first loop, a second electrical loop includingfirst switch means having open and closed conditions and positioned foroperation by said first switch operating means in said first loop,second switch operating means in series with said first switch means insaid second loop, a third electrical loop including heating means forheating said mass, second switch means in series with said heating meansand having open and closed conditions and positioned for operation bysaid second switch operating means, said first switch operating meansbeing responsive to variations in electrical current flowing throughsaid variable resistance means for a cyclically operating said firstswitch means, and said second switch operating means being responsive tocyclical operation of said first switch means for cyclically operatingsaid second switch means to control operation of said heating means. 2.The circuit of claim 1 wherein said first switch operating meanscomprises a first resistance heater and said first switch means includesa first bimetallic element positioned in heat transfer relationship withsaid first resistance heater.
 3. The circuit of claim 2 wherein saidsecond switch operating means comprises a second resistance heater andsaid second switch means includes a second bimetallic element positionedin heat transfer relationship with said second resistance heater.
 4. Thecircuit of claim 3 and further including adjustable control means foradjusting the temperature at which said first switch means is cyclicallyoperated by said first switch operating means.
 5. The circuit of claim 1wherein said second switch operating means comprises a second resistanceheater and said second switch means includes a second bimetallic elementpositioned in heat transfer relationship with said second resistanceheater.
 6. The circuit of claim 1 and further including adjustablecontrol means for adjusting the temperature at which said first switchmeans is cyclically operated by said first switch operating means.
 7. Atemperature control device for controlling operation of heating meansfor heating a predetermined mass to a desired temperature comprising;automatically variable resistance means having automatically variableresistance characteristics with varying temperature for passingelectrical current therethrough at a value proportional to itstemperature, said variable resistance means being positioned in heatexchange relationship with said mass, first switch operating means inseries with said variable resistance means, first switch means havingopen and closed conditions and being positioned for selective operationbetween said open and closed conditions by said first switch operatingmeans, second switch operating means in seRies with said first switchmeans for operation in response to opening and closing of said firstswitch means, second switch means having open and closed conditions andbeing positioned for selective operation between said open and closedconditions by said second switch operating means, said second switchmeans being connected in series with said heating means.
 8. The deviceof claim 7 wherein said first switch operating means comprises anelectro-magnetic coil and said first switch means comprises amagnetically-operated switch.
 9. The device of claim 7 wherein saidfirst switch operating means comprises a first resistance heater andsaid first switch means includes a first bimetallic element positionedin heat transfer relationship with said first resistance heater.
 10. Thedevice of claim 7 wherein said second switch operating means comprises asecond resistance heater and said second switch means includes a secondbimetallic element positioned in heat transfer relationship with saidsecond resistance heater.